To test the hypothesis that the movement of water and protein across the canine visceral pleura is accelerated by conditions which augment the transcapillary movement of fluid in the lung, the transfer of these substances will be quantitatively analyzed in terms of a two-compartment model of fluid exchange in which the endothelial-pleural barrier to fluid movement is considered a single semi permeable membrane. Utilizing a new technique to isolate the entire filtrate of the visceral pleura, the hydrostatic and oncotic pressures on each side of the membrane and the actual rate of protein and water movement between the intravascular and pleural compartments will be measured in spontaneously breathing dogs and from this data the hydraulic conductivity (Lp) and the permeability and reflection coefficients for protein (Omega, Sigma) of the idealized visceral pleural membrane will be calculated. To confirm tht the Lp, Omega and Sigma of the idealized endothelial-pleural membrane are valid quantitative estimates of the visceral pleural capillaries'LP, Omega and Sigma, it will be shown that lymphatic ligation which increases the water content and hydrostatic pressure of the interstitial compartment of the lung does not alter water or protein movement across the pleural membrane. These parameters will be measured in various pathological conditions to determine if the increased water and protein movement observed in these states is due to alterations in membrane permeability or simply a reflection of abnormal hydrostatic and oncotic gradients across the membrane. If Lp, Omega or Sigma are significantly altered by a) extremely high (Greater than 40 mm Hg) pulmonary capillary pressures, b) extremely low (Less than 2 gm percent) plasma protein concentrations, or c) the infusion of a pulmonary endothelial toxin (oleic acid), the ability of fibronectin, a potential modulator of endothelial integrity, to correct the abnormalities will be examined. Since the capillary endothelium in the canine visceral pleura is derived form the pulmonary circulation, fluid flux across the visceral pleura reflects the rate of solute and water movement across the pulmonary capillaries. A quantitative analysis of fluid exchange across the visceral pleura will enable us to better undestand the dynamics of pleural fluid formation; it may also allow us to more clearly delineate the movement of solute and water in the lung.